The creation and emission of vapor-phase gas pollutants can be due to many human activities. For example, such pollutants can result from fuel combustion for energy, including fossil fuels, landfill gases and wood gasifier product gas. Combustion is also used for disposal of waste products which can be burned. Such waste products include gas from solid waste landfills (landfill gas), waste gas from oil refineries, and solid waste disposed of in incineration processes. Air streams or process exit gas can also commonly be contaminated with volatile organic compounds (VOC's), e.g. by cooking, fires, emissions of aerosols or their propellants, or putrescing wastes.
The combustion of fuels for energy, or burning of materials for disposal results in contaminants in product gas that include oxides of nitrogen, carbon monoxide, volatile organic compounds, and sulfur oxides. Among contaminants of regulatory concern are most often the three major oxides of nitrogen: nitrous oxide (N2O), nitric oxide (NO) and nitrogen dioxide (NO2) and, as well, carbon monoxide. As one example, U.S. emission limit standards commonly applied to natural gas firing are 0.06 pounds of NOx per one million Btu's and 0.2 pounds of CO (carbon monoxide) per million Btu's.
Internal combustion (IC, or piston) engines have a number of attractions, including their ability to use many fuels, such as landfill gas, and ready maintenance. But, despite their numerous advantages, such engines emit nitrogen oxides in amounts that are about five-fold those of other combustion-based mechanical and electrical power options. Depending on fuels and operations, IC Engines can also emit CO and unburned higher (C2+ [two-carbon] and up) hydrocarbons. Unburned higher hydrocarbons are then emitted as VOC's, which are regulated local air pollutants. In addition, the combustion of fuels containing sulfur can give rise to sulfur oxides (SOx). Even as needs for various combustion processes increase in the US and worldwide, regulation and restriction of emissions, particularly NOx, and CO, is becoming so stringent as to limit the use of these processes to far less than their full potential.
If these contaminant emissions could be reduced to meet regulatory standards (generally state or district-specific), or even better, eliminated, then the use of some combustion processes would be feasible both from economic, environmental and regulatory standpoints. For example, solid waste landfills generate combustible gas, which is undesirable if released into the atmosphere because of the presence of methane and higher hydrocarbon contaminants. However, if flue or exhaust gases resulting from landfill gas combustion can be appropriately cleaned, the landfill gas can be more readily used as a renewable fuel through its combustion in energy generating plants. This use not only abates pollutants, but can also reduce atmospheric emissions of landfill methane, a gas whose emissions are considered in the aggregate to have major adverse climate effects. Such landfill gas fuel use can conserve other fuels as well as reduce emission of fossil CO2 from lessening use of fossil energy sources. For this reason, landfill gas energy use or abatement has substantial climate benefits. Benefits associated with energy use are such that the U.S. EPA encourages landfill gas use in its very active Landfill Methane Outreach Program (LMOP)
Using landfill gases or other energy sources for electricity generation at multiple smaller-scale sites near population centers, has become increasingly desirable, because it substantially lessens congestion and resultant resistance losses of electric power, especially in longer-distance electrical transmission lines. Such electricity fueling over multiple widespread facilities is termed “distributed generation”. As noted, landfill gas combustion, whether for powering internal combustion engines or for use in other fuel combustors, can increase air pollutant emissions, particularly nitrogen oxides and carbon monoxide. Current emissions standards severely constrain use of landfill gas, in fueling such distributed generation. Thus methods for reduction or avoidance of emissions will be of high value.
Simultaneously, even as landfill gas energy use and distributed generation is emission constrained, landfill management technology is advancing concepts showing superior environmental benefits and potentials—including realization of much-increased landfill gas energy, if energy-related emissions were not barriers (Pacey et al. (1999) The Bioreactor Landfill—An Innovation in Solid Waste Management. Solid Waste Association of North America, Silver Spring, Md.)
The present Federal subtitle D landfill regulations evolved with the goal of keeping waste landfills dry. (This has, in the past decade, become known as the “Dry Entombment” approach). Alternative strategies have been recognized for some time and are now being rapidly developed. It is being shown that waste decomposition and methane generation can be accelerated and be better controlled by improving bacterial reaction conditions in “bioreactor” landfills (see, for example, Augenstein et al. (1976); Fuel Gas Recovery from Controlled Landfilling of Solid Wastes, Resources and Conservation, 1, 103-117; Barlaz et al. (1990); Methane Production from Municipal Refuse—A Review of Enhancement Techniques and Microbial Dynamics, Critical Reviews in Environmental Control, 19(6):557-584; Stessel et al. (1994) Design Implications of the in-Ground Digester, Proceedings, Air and Waste Management Association Meeting Cincinnati, June 19-24; Augenstein et al. (2000) Yolo County Controlled Landfill Project June, Proceedings, Second International Methane Mitigation Conference, Akademgorodok, Novosibirsk, Siberia, Russia. Proceedings available from US EPA. The bioreactor approach has a number of benefits as seen by landfill owner/operators and also regulators (see Pacey et al. (1999), supra.)
Yet other processes of numerous types are limited by emissions of pollutant or odorous compounds or both. There are a host of these processes, e.g. animal husbandry, confined animal operations, aerobic composting, forcing air through landfills for composting or heating purposes, and the like.
The present invention provides methods that reduce undesirable contaminating gases from combustion or landfill gas fueled generation, or polluted/odorous gases from other sources. A substantial source of renewable energy can be freed for economical use, and polluted exhaust gas or other gas streams can be cleaned at a relatively low cost.
Relevant Literature
Patents relating to bioremediation of gaseous pollutants include U.S. Pat. No. 5,503,738 (Apr. 2, 1996), which describes a process for remediating vaporous pollutants by passage through a bioreactor containing microorganisms capable of remediating the pollutants. U.S. Pat. No. 5,795,751 (Aug. 18, 1998) discloses a biofilter for reducing concentrations of gaseous nitrogen oxides in a polluted gas through an organic filter bed with denitrifying bacteria. U.S. Pat. No. 6,013,512 (Jan. 11, 2000) provides an apparatus for scrubbing gaseous emissions with simultaneous liquid scrubbing and biochemical decomposition of NOx or VOC or a combination thereof using an aqueous suspension held in a biomass chamber. U.S. Pat. No. 6,117,672 (Sep. 12, 2000) provides a system combining biomass filtration, anaerobic digestion, steam absorption refrigeration and heat exchangers wherein moist waste consumes the “nitrogenous oxides” in gases from a fuel combustor.
Other relevant work has been sponsored, for example by the California Air Resources Board, in (Hudepohl. N. J., Davidova, Y. du Plessis. C. Schroeder, E. D., and Chang, D. P. Y. 1999 Biofilter Technology for NOx Control. Department of Civil and Environmental Engineering, University of California, Davis). This uses a column reactor and nitrifying organisms to convert NO to NO2, thence nitrate.
Patents relating to aerobic landfills include U.S. Pat. No. 5,546,862, relating to a method of landfill mining. U.S. Pat. No. 5,888,022 is directed to methods of improving aerobic degradation of the solid wastes emplaced in a landfill. U.S. Pat. No. 6,024,513 discloses improved methods of decomposing municipal solid wastes placed in a landfill.